This invention relates generally to the field of optical communications and in particular to dispersive optical devices for use in wavelength division multiplexing, in optical spectrum analysis, and in optical signal processing.
In optical communications systems, a plurality of data channels may be transmitted simultaneously over an optical fiber wherein each data channel is characterized by a separate wavelength, or frequency. At the transmission end of the fiber, a set of diode lasers, each operating at a separate wavelength, establishes a corresponding set of data channels that are subsequently combined into a common optical path by means of wavelength division multiplexing (WDM). In WDM systems, a multiplexer is operative to receive a plurality of data channels each having a separate wavelength and to combine and transmit the data channels to a single optical fiber. At the receiving end of the fiber, a multiplexer can be used in reverse to demultiplex, or separate, the data channels according to their respective wavelengths, so that the data corresponding to each wavelength channel can be guided to its own detector or optical fiber.
WDM devices have been developed using a broad variety of technologies, including diffraction gratings, interference filters, Mach-Zehnder interferometers, Fabry-Perot etalons, birefringent elements, and prism configurations. Diffraction gratings can disperse incident light with high resolution, so that in the grating output the diffracted angle is a function of wavelength. When a beam that comprises a plurality of wavelengths incident to a diffraction grating, the beam is diffracted into sub-beams that can be focused by a lens onto a set or an array of detectors that receive sub-beams having different wavelengths.
Diffraction gratings are widely used to disperse light into its spectral components for measurement of the spectral content of an optical beam or signal. In many cases it is desirable or necessary that the grating provide a maximum amount of dispersion; this can be accomplished by using a grating that has a high groove density.
In certain applications, it is desirable that the grating diffract incident light with the same efficiency regardless of the polarization state of the incident light. However, it is well known that the diffraction efficiency of gratings in general is not constant for different polarization states of the incident light. Typically gratings are analyzed in terms of the diffraction efficiency of light that is polarized perpendicular to the plane of incidence (s-polarization) and of light that is polarized parallel to the plane of incidence (p-polarization). A light beam of an undefined polarization state may be analyzed in terms of the s- and p-polarization components of the beam. In particular, reflective surface-relief gratings, which are the type of gratings in widest use, exhibit a significant difference in the efficiency with which they diffract the different polarizations.
This invention resides in an optical element capable of separating an optical signal made up of multiple wavelengths into separate optical signals, each constituting a single wavelength channel. Such a device can therefore serve as a wavelength demultiplexer or optical spectrum analyzer in fields such as optical communications and optical signal processing. In addition, the device can function in a reverse sense, as a wavelength multiplexer, for example, to combine a plurality of optical signals each of a different wavelength, into a single optical signal.
The optical element preferably comprises a volume phase grating supported between substrates and prisms. The prisms facilitate the use of input and output beams that make large angles with respect to the normal to the plane of the grating. In a configuration with large input and output angles, the grating provides a very high degree of dispersion, which leads to improved separation of closely spaced wavelength channels. In addition, the average refractive index of the grating medium is less than that of the supporting substrates and prisms, in which case the optical element provides improved uniformity and reduced sensitivity to the state of polarization of the input beam over a range of wavelengths.
The grating may be constructed by conventional interferometric or holographic techniques, and may be a reflection or transmission device, though the latter is used in the preferred embodiment. In a system configuration, optical fibers may be used to carry the multiplexed or demultiplexed optical signals. Optoelectric detectors may also be used to detect different wavelengths and convert the optical signals into electrical counterparts. Alternatively, electrical signals may be converted to optical signals of differing wavelength, and these may be multiplexed using one or more of the inventive devices. Of course, optical fibers, optoelectric detectors, and emitters may be used in various combinations along with the inventive optical device depending upon the application.